Dark matter swirling into a black hole?

If Dark Matter exists, and the only way normal matter interacts with it
is through gravity, then shouldn't there be Dark Matter vortex swirling
into one of the biggest sources of normal matter mass around, i.e. a
black hole? In fact, shouldn't the black hole be getting bigger by
several fold, just by fattening up with Dark Matter, which is supposed
to be an order of magnitude more prevalent than Normal Matter? A black
hole would then be growing more massive even if no sources of normal
matter are nearby to feed it.

Yousuf Khan

Dear Yousuf Khan:

"Yousuf Khan" wrote in message
...
If Dark Matter exists, and the only way normal matter
interacts with it is through gravity, then shouldn't there
be Dark Matter vortex swirling into one of the biggest
sources of normal matter mass around, i.e. a black hole?

Black holes are not *sources* of normal matter... until they
evaporate.

In fact, shouldn't the black hole be getting bigger by several
fold, just by fattening up with Dark Matter,
which is supposed to be an order of magnitude more
prevalent than Normal Matter? A black hole would
then be growing more massive even if no sources
of normal matter are nearby to feed it.

But we can only detect black holes by the interactions in
infalling matter with other bits of infalling matter. Dark
matter would have no such interactions. This would make a Dark
Black Hole hard to spot!

David A. Smith

N:dlzc D:aol T:com (dlzc) wrote:
Dear Yousuf Khan:

"Yousuf Khan" wrote in message
...
If Dark Matter exists, and the only way normal matter
interacts with it is through gravity, then shouldn't there
be Dark Matter vortex swirling into one of the biggest
sources of normal matter mass around, i.e. a black hole?

Black holes are not *sources* of normal matter... until they
evaporate.

Alright then, I should've said, "made from normal matter". Basically
once matter falls into a blackhole, it's just a bunch of mass, so it
shouldn't matter if its mass is coming from normal matter or dark
matter, it's all just mass. However, a black hole's mass only seems to
correspond to the amount of normal matter that's fallen into it. Why
isn't its mass growing even fatter due to dark matter falling into it?

But we can only detect black holes by the interactions in
infalling matter with other bits of infalling matter. Dark
matter would have no such interactions. This would make a Dark
Black Hole hard to spot!

But that's not the point. The point is why doesn't a black hole grow
fatter with dark matter? Let's say normal matter is falling into some
blackhole and we can therefore detect its presense. When the hole's mass
is computed, it only corresponds to the amount of normal matter mass
that's fallen in. It's mass should be much higher due to dark matter
increasing its mass even more than whatever amount of normal matter is
falling in.

Yousuf Khan

Ok... a quick and dirty answer is that even with an overestimate of
the mass density of posited 'dark matter' in our galaxy, its rate of
accretion into a stellar mass black hole would add only a minuscule
fraction to the black hole's mass. Let's say the dark matter mass
density is 10 times that of normal luminous mass density. That might
make it about 10^-17 or 10^-18 kg / meter^3 (way too high I think,
but we're trying to find an upper bound). Another poster correctly
pointed out that unless the stuff were falling directly into the black
hole, it would mostly just fly on by, since it has NO means of losing
energy or angular momentum through normal dissipation processes.
A very inefficient accretion source.

So, lets pretend that the black hole can at least engulf a quantity of
dark matter that would inhabit a spatial volume equal to the volume of
the black hole. A ten solar mass BH has a volume of ~10^15 meter^3.
Let's also pretend that it engulfs this amount of dark stuff every
second.
That gives about 10^-4 kg per second or equal to or less than 10^15 kg
over the _entire age_ of the universe. This is obviously only a tiny
fraction of the BH's actual mass.

cheers

Dear Yousuf Khan:

"Yousuf Khan" wrote in message
...
N:dlzc D:aol T:com (dlzc) wrote:
....
But we can only detect black holes by the interactions
in infalling matter with other bits of infalling matter.
Dark matter would have no such interactions. This
would make a Dark Black Hole hard to spot!

But that's not the point. The point is why doesn't a black
hole grow fatter with dark matter?

If we can't see it, or see haven't seen it grow, how could we
know if what you suggest was or was not happening?

Let's say normal matter is falling into some blackhole
and we can therefore detect its presense. When the
hole's mass is computed, it only corresponds to the
amount of normal matter mass that's fallen in.

A black hole's event horizon is established by the *amount* of
matter it contains... not the type of matter. Only net charge,
net mass, and net angular momentum are available outside the BH.
No knowledge of what would make matter Dark... since it is now
*all* Dark.

It's mass should be much higher due to dark matter increasing
its mass even more than whatever amount
of normal matter is falling in.

No. Matter is just matter where BHs are concerned.

David A. Smith

"N:dlzc D:aol T:com (dlzc)" wrote in message ...
| Dear Yousuf Khan:
|
| "Yousuf Khan" wrote in message
| ...
| If Dark Matter exists, and the only way normal matter
| interacts with it is through gravity, then shouldn't there
| be Dark Matter vortex swirling into one of the biggest
| sources of normal matter mass around, i.e. a black hole?
|
| Black holes are not *sources* of normal matter... until they
| evaporate.

Hahahaha!

When you find a black hole, Smith, look inside and tell
me if there is any broken eggshell inside, please. Bright
green flying elephants have always used black holes
for their aeiries. How many bright green flying elephants
can dance on the accretion disk of a hole?

Yousuf Khan writes:
If Dark Matter exists, and the only way normal matter interacts with
it is through gravity, then shouldn't there be Dark Matter vortex
swirling into one of the biggest sources of normal matter mass around,
i.e. a black hole? In fact, shouldn't the black hole be getting bigger
by several fold, just by fattening up with Dark Matter, which is
supposed to be an order of magnitude more prevalent than Normal
Matter? A black hole would then be growing more massive even if no
sources of normal matter are nearby to feed it.

Getting a black hole to "suck up" even normal matter is actually
rather difficult. Of course there will be a very small fraction of
gas and stars that free-fall directly into the black hole. However,
most will not. Most normal matter in our galaxy is not gravitational
bound to the black hole in the center. The tiny fraction that is
bound probably got that way via tidal interactions and frictional
dissipation, which then forms an accretion disk. Even then, it is
fairly difficult to push matter into the black hole, since a large
amount of potential energy and angular momentum must be removed.
However, accretion disks are highly viscuous and turbulent, and a
small amount of mass transport does occur.

Looking at the mechanisms quoted above for binding matter to a black
hole and pushing it in: "tidal interactions" - "frictional
dissipation" - "accretion disk" - "viscuous" - "turbulent".
None of these effects is known to apply to dark matter since then
all require normal matter (electromagnetic) interactions.

Thus, most dark matter near a black hole probably spends its time in
bound orbits quasi-forever, without dissipation. And probably an even
larger fraction of far away dark matter is not even bound to the black
hole and does whatever it does.

CM

In article ,
Craig Markwardt writes:
Thus, most dark matter near a black hole probably spends its time in
bound orbits quasi-forever, without dissipation.

Consider also that the _density_ of dark matter is very low indeed.
Even though dark matter comprises most of the mass of the Universe,
that mass is spread over a huge volume.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

On Jan 2, 10:16 pm, Craig Markwardt
wrote:
Getting a black hole to "suck up" even normal matter is actually
rather difficult. Of course there will be a very small fraction of
gas and stars that free-fall directly into the black hole. However,
most will not. Most normal matter in our galaxy is not gravitational
bound to the black hole in the center. The tiny fraction that is
bound probably got that way via tidal interactions and frictional
dissipation, which then forms an accretion disk. Even then, it is
fairly difficult to push matter into the black hole, since a large
amount of potential energy and angular momentum must be removed.
However, accretion disks are highly viscuous and turbulent, and a
small amount of mass transport does occur.

Looking at the mechanisms quoted above for binding matter to a black
hole and pushing it in: "tidal interactions" - "frictional
dissipation" - "accretion disk" - "viscuous" - "turbulent".
None of these effects is known to apply to dark matter since then
all require normal matter (electromagnetic) interactions.

So, Dark Matter doesn't interact with itself anymore than it interacts
with Normal Matter?

Yousuf Khan

"bbbl67" writes:
On Jan 2, 10:16 pm, Craig Markwardt
wrote:
Getting a black hole to "suck up" even normal matter is actually
rather difficult. Of course there will be a very small fraction of
gas and stars that free-fall directly into the black hole. However,
most will not. Most normal matter in our galaxy is not gravitational
bound to the black hole in the center. The tiny fraction that is
bound probably got that way via tidal interactions and frictional
dissipation, which then forms an accretion disk. Even then, it is
fairly difficult to push matter into the black hole, since a large
amount of potential energy and angular momentum must be removed.
However, accretion disks are highly viscuous and turbulent, and a
small amount of mass transport does occur.

Looking at the mechanisms quoted above for binding matter to a black
hole and pushing it in: "tidal interactions" - "frictional
dissipation" - "accretion disk" - "viscuous" - "turbulent".
None of these effects is known to apply to dark matter since then
all require normal matter (electromagnetic) interactions.

So, Dark Matter doesn't interact with itself anymore than it interacts
with Normal Matter?

I'm not aware of any evidence that it does. Presumably all matter
(dark and normal) interacts via gravity, which is not dissipative.
Normal matter can also interact via the electromagnetic force (and
thereby radiate energy away), but dark matter cannot, by definition.

CM